RPM vs. MPG
Thread Starter
Le Mans Master
Joined: Sep 2000
Posts: 5,635
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From: Orlando Floriduh
RPM vs. MPG
Anyone else have this experience or am I going crazy? My 69 BB (stock l36 with 3.55:rear) gets Better mileage at 75-80 (3400-3500 rpm) than it does at 60. I had a 70-454 that did the same thing. I also had a 92 700r4 that got the same mileage in 3rd gear (per computer read-out) as in OD until speeds got above 75 and even then the mileage gain was only tenths better. 
Le Mans Master
Joined: Mar 2004
Posts: 7,502
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From: Little Rock AR
Your engine has greater efficiency at higher RPM. It is cammed that way. The cam causes the efficiency to move around the RPM band. You are seeing the effects of the car operating in the more efficient range. You could drop the rear end gearing and see the efficency go up at lower road speeds.
-Mark.
-Mark.
Melting Slicks
Joined: Feb 2003
Posts: 3,053
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St. Jude Donor '03,'11
Originally Posted by ZD75blue
What I wonder is how those german guys can afford to run the audobahn!
Go out and do 150mph... that'll suck some gas!
Go out and do 150mph... that'll suck some gas!
Most of them don't go that far though, its a tiny country.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
Wonder if your carb is running rich at lower rpm? On my 71 I went from a 4-speed and ~3000 rpm cruise to a 5-speed and ~2000 rpm cruise. My mpg went from 15 with the 4-speed to 21 with the 5-speed.
Safety Car
Joined: Oct 2004
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From: Youngstown Ohio
Originally Posted by Larry B.
Anyone else have this experience or am I going crazy? My 69 BB (stock l36 with 3.55:rear) gets Better mileage at 75-80 (3400-3500 rpm) than it does at 60. I had a 70-454 that did the same thing. I also had a 92 700r4 that got the same mileage in 3rd gear (per computer read-out) as in OD until speeds got above 75 and even then the mileage gain was only tenths better.
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Race Director
Joined: Dec 1999
Posts: 11,096
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From: Sierra Foothills CA
That's pretty interesting. Our Vettes have alot of basic things in common although I don't know how stock your L36 is. I've never tried highway cruising faster than 65-70 MPH so I can't compare MPGs- I can tell you that if I hit the mid-teens on a trip I'm doing very well. 
Drifting
Joined: Feb 2002
Posts: 1,906
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From: Berlin
Fuel economy depends on throttle position more than rpm. As others have said, your engine becomes more efficient at higher rpm, so it requires less throttle. This compensates for the increased rpm.
Although I've never checked gas mileage very carefully, both of my cars require much less throttle at 80 than 60.
Although I've never checked gas mileage very carefully, both of my cars require much less throttle at 80 than 60.
Race Director
Joined: Apr 1999
Posts: 11,520
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From: CORVETTE 77 385 C.I. TEXAS
Originally Posted by Larry B.
Anyone else have this experience or am I going crazy? also had a 92 700r4 that got the same mileage in 3rd gear (per computer read-out) as in OD until speeds got above 75 and even then the mileage gain was only tenths better.
As zwede implies some tuning might help.
Have seen the same thing w/ O/D trans.
Tenths- see CAFE (Corporate Average Fuel Economy)- the fine was
No. of tenths over X $500 X no. of vehicles produced.
So GM was watching those tenths very closely & the mpg tests were inside so no wind resistance ...Larry
So you're not crazy (yet anyway).
Race Director
Joined: Sep 1999
Posts: 11,361
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From: Plano TX
Originally Posted by Langadorf
Have you run your car on a dyno? At what RPM do you hit peak torque? That's where you engine is most efficient.
For fuel economy you would be looking at BSFC (brake specific fuel consumption). BSFC tells you how much fuel you need to produce each horsepower. BSFC will go down (=better) at lower rpm until the engine is lugging where it goes up again.
Le Mans Master
Joined: Aug 2003
Posts: 7,720
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From: Chattanooga TN
Originally Posted by stingr69
Your engine has greater efficiency at higher RPM. It is cammed that way. The cam causes the efficiency to move around the RPM band. You are seeing the effects of the car operating in the more efficient range. You could drop the rear end gearing and see the efficency go up at lower road speeds.
-Mark.
-Mark.
Le Mans Master
Joined: Feb 1999
Posts: 6,807
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From: Hoosier
Cruise-In III Veteran
I don't disagree with you guys but bear in mind that carburater settings do make a difference. At light throtle and higher engine vacuum the idle circuit is drawing fuel. On mine (very mild almost stock 350) I believe the throtle blades are still within the transistion slot pulling that extra fuel.
Thread Starter
Le Mans Master
Joined: Sep 2000
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From: Orlando Floriduh
Originally Posted by AlwaysWave
I don't disagree with you guys but bear in mind that carburater settings do make a difference. At light throtle and higher engine vacuum the idle circuit is drawing fuel. On mine (very mild almost stock 350) I believe the throtle blades are still within the transistion slot pulling that extra fuel.
Thread Starter
Le Mans Master
Joined: Sep 2000
Posts: 5,635
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From: Orlando Floriduh
I love the 3.55's Plenty of power.... 3000rpm @ 70mph At 75-80 I get 16-17 but less at 60. In town.... Don't ask That is with Real lead additve to give me 100 octane. With 93 Amoco( no additve) mileage is a lot less. Like 13 at best plus it pings in warm weather with the A/C.
That is with Real lead additve to give me 100 octane. With 93 Amoco( no additve) mileage is a lot less. Like 13 at best plus it pings in warm weather with the A/C.
